Journal of Geoscience and Environment Protection, 2014, 2, 134-142
Published Online April 2014 in SciRes. http://www.scirp.org/journal/gep
http://dx.doi.org/10.4236/gep.2014.22019
Prediction of CO2 and H2S Emissions from
Wastewater Wet Wells
Madhuri Mudragaddam, Bhaskar Kura, Amrita Iyer, Elena Ajdari
Civil & Environmental Engineering, University of New Orleans, Louisiana, USA
Email: [email protected]
Received February 2014
Abstract
The transportation of wastewater through sewer networks can cause potential problems due to
the formation of lethal gases such as ammonia, carbon dioxide, hydrogen sulphide and methane.
As a result, they can cause irritating, toxic, and microbial induced corrosion. The true depth of
these problems can be measured by measuring the emission of such lethal gasses in the sewer atmosphere. The amount of gases released from such sewer networks has to be controlled. In this
paper, we present a simple experimental methodology used to determine the emissions of Carbon
Dioxide (CO2) and Hydrogen Sulphide (H2S) using Landtec GEM-2000 plus multi-gas analyser from
wastewater wet wells.
Keywords
Wastewater Lift Station; Hydrogen Sulfide; Carbon Monoxide; Air Pollutants
1. Introduction
The sewage collection stations, commonly known as a Lift Station, are designed to pump raw sewage that comes
from underground gravity pipelines. This raw sewage is accumulated in the collection pit or tank known as a wet
well. Electrical level indicator is located in the wet well to determine the instantaneous accumulation of raw sewage. When these level indicators trip, they energize pumps to evacuate the raw sewage through a pressurized
pipe system called a sewer force main. Form this stage, it is transferred to sewage discharge and a gravity manhole. The cycle is repeated herein until the sewage reaches the treatment plant.
The process in which wastewater is collected, conveyed or treated has the potential to generate and release
several gases to the surrounding area that are asphyxiating, irritating, toxic or flammable. These include ammonia (NH3), carbon dioxide (CO2), carbon monoxide (CO), hydrogen sulfide (H2S) and methane (CH4). This paper’s main focus is only on CO2 and H2S emissions from the wastewater wet wells
1.1. Carbon Dioxide
Carbon Dioxide (CO2) is a colorless, odorless, non-flammable gas and is the most prominent Greenhouse Gas
(GHG) in Earth’s atmosphere. It is recycled through the atmosphere by the process photosynthesis, which makes
human life possible. It is an asphyxiant. When carbon dioxide accumulates to more than 10% of the atmosphere,
How to cite this paper: Mudragaddam, M. et al. (2014). Prediction of CO2 and H2S Emissions from Wastewater Wet Wells.
Journal of Geoscience and Environment Protection, 2, 134-142. http://dx.doi.org/10.4236/gep.2014.22019
M. Mudragaddam et al.
narcosis may occur in individuals breathing the carbon dioxide. Carbon dioxide is heavier than air and collects
in the lower levels of confined spaces (Gerardi Michael & Zimmerman Mel, 2005). Under anaerobic conditions
carbohydrates are converted into acetate, hydrogen and carbon dioxide, hence CO2 is released from wet wells.
Human activities such as the production and consumption of fossil fuels, as well as agricultural and industrial
activities have caused an increase in the atmospheric concentration of harmful GHGs, particularly CO2, CH4,
and N2O (El-Fadel & Massoud, 2001). The increase in atmospheric GHG concentration has led to climate
change and global warming effect. The contribution of a GHG to global warming is commonly expressed by its
global warming potential (GWP), which enables the comparison of global warming impact of a gas and that of a
reference gas, typically carbon dioxide. On a 100-year basis, the GWP of carbon dioxide, methane and nitrous
oxide are 1, 23, and 296, respectively (IPCC, 2001).
1.2. Hydrogen Sulfide
In most instances, the odors associated with collection systems and primary treatment facilities are generated as
a result of an anaerobic or “septic” condition. This condition occurs when oxygen transfer to the wastewater is
limited such as in a force main. In the anaerobic state, the microbes present in the wastewater have no dissolved
oxygen available for respiration. This allows microbes known as “sulfate-reducing bacteria” to thrive. These
bacteria utilize the sulfate ion (SO4-) that is naturally abundant in most waters as an oxygen source for respiration. The byproduct of this activity is hydrogen sulfide (H2S). This byproduct has a low solubility in the wastewater and a strong, offensive, rotten-egg odor. In addition to its odor, H2S can cause severe corrosion problems.
(Palmer et al., 2000) Due to its low solubility in the wastewater, it is released into the atmosphere in areas like
wet wells, head works, grit chambers and primary clarifier. There are typically other “organic” odorous compounds, such as mercaptans and amines, present in these areas, but H2S is the most prevalent compound
(Vaughan Harshman & Tony Barnette, 2000).
The distinctive odor (rotten egg smell) of H2S gas is easily perceptible at very low levels (<1 ppm in air). Exposure to moderate H2S levels (10 - 50 ppm) can cause headaches, dizziness, nausea and vomiting, coughing
and breathing difficulty. Exposure to H2S Levels above 100 ppm can result in severe respiratory tract and eye irritation and extreme cases even death (US Department of Health Education and Welfare, Public Health Service,
National Institute for Occupational Safety and Health, Cincinnati).
2. Details of the Lift Station
2.1. Name: Wet Well #1
The station is a three pump, flooded suction type station. The station is enclosed in block wall superstructure, and
it consists of a rectangular wet well, approximately 36 ft long, 20.5 ft wide and 18.67 ft deep as shown in Figure 1.
The pump capacities of this lift station with 100% valve opening are
Pump 1: 11042 gpm @ 47 ft of total dynamic head
Pump 2: 6771 gpm @ 50 ft of total dynamic head
Pump 3: 6601 gpm @ 48 ft of total dynamic head
Both when pump 1 and pump 2 are running with 100% valve opening, the capacity of the lift station is 14665
gpm at 50 ft of total dynamic head.
2.2. Name: Wet Well #2
The lift station is a two pump, suction lift type station. The station has no superstructure, but wooden fencing
with locking gate and it consists of a circular wet well with a circular shaft. The dimension of the circular shaft
is 4 ft in diameter and 9 ft deep. The actual wet well is 9 ft in diameter and 7.42 ft deep as shown in Figure 2.
When both the pumps are running the capacity of the lift station is 1300 gpm @ 44 ft of total dynamic head.
3. Landtec GEM 2000 Plus Instrument
The GEM 2000 Plus instrument designed by Landtec is a landfill gas analyzer which samples and analyzes methane, carbon dioxide and oxygen content in percentage and carbon monoxide and hydrogen sulfide in parts per
million. The gas readings are displayed and can be stored in the instrument and downloaded to a personal com-
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Figure 1. Schematic diagram of a wet well (Site 1).
Figure 2. Schematic diagram of a wet well (Site 2).
puter for reporting, analyzing and archiving. The best thing about this instrument is the data logging function
which it has. The time interval between the readings can be set in the instrument, and the instrument will take
the readings by turning on and off itself to the time interval specified. This instrument measures CO2 in percent
(%) which has to be converted to parts per million by multiplying the percentage with 104. The H2S readings are
measured directly in ppm by the instrument. The range of CO2 and H2S for this instrument are 0% - 100% and 0 - 500
ppm respectively.
4. Methods to Control H2S Emissions Wastewater
Listed below are few methods available to control the rate of hydrogen sulfide corrosion.
1) Reducing the dissolved sulfide content of the wastewater
2) Using corrosion-resistant materials and coatings
3) Providing ventilation of the enclosed area of the sewer and
4) Conducting routine preventive maintenance.
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Three basic techniques to control dissolved sulfide:
a) Oxidation by the addition of chemical oxidants such as hydrogen peroxide, chlorine or potassium permanganate, or by the introduction of air or oxygen.
b) Precipitation of dissolved sulfide by the addition of metallic salts such as ferrous chloride and ferrous sulfate.
c) pH elevation through the addition of sodium hydroxide. Since low pH is favorable to produce H2S gas, so it
has to be increased (EPA, 1991).
5. Methodology
Wastewater during its stay in a wet well produces CO2 and H2S gas. When the volume of wastewater increases
in the well these gases are expelled out. Landtec GEM-2000 Plus, a CO2 and H2S detecting instrument is used to
measure these gases inside the wet well. This instrument gives the concentration of CO2 in percentage and H2S
in ppm (parts per million). The percentage measure is converted into ppm by multiplying it by 104 (0.1% = 1000
ppm). One ppm here means one part of CO2 or H2S is present in one million parts of air.
As the volume of the wastewater in the wet well increases, these gases are pushed out, when the volume of
wastewater decreases the surrounding ambient air is dragged into the well to fill the water column with the air.
Hence the concentrations in the air volume of the wet well changes depending upon the water level in the well.
Steps for Calculation:
CO2 and H2S emissions from the wet well in terms of pounds can be calculated as below:
CO2 expelled out of the wet well (lb) = Volume of Air Expelled Due to Increase in the water level *Average
Hourly Concentration of CO2/H2S in the Wet Well * CO2/H2S density
Assuming CO2 density at standard temperature and pressure = 1.98 kg/m3 = 0.124 lb/ft3
The density of H2S is 1.363 kg/m3 equal to 0.0851 lb/ft3.
CO2 and H2S emissions from the wet well in terms of pounds per cubic feet can be calculated as below:
CO2 or H2S emitted per unit volume of wastewater (lb/ft3)
= mass of CO2 or H2S emitted/average hourly volume of wastewater present in the wet well
= mass of CO2 or H2S/[(V1 + V2)/2]
where
V1 = Initial volume of wastewater present in the wet well
V2 = Final volume of wastewater present in the wet well.
If a person takes a gas (CO2 or H2S) measurement at 8 A.M. and another gas measurement at 9 A.M., the
Volume of wastewater present in the wet well at 8 A.M. is considered as the initial volume of wastewater and
the volume of wastewater present in the wet well at 9 A.M. is considered as the final volume of the wastewater.
If that person takes another gas measurement at 10 A.M., for the next calculation the volume of wastewater present in the wet well at 9 A.M. will become initial volume of wastewater present in the wet well and the volume
of wastewater present in the wet well at 10 A.M. will be the final volume of wastewater present in the well.
5.1. Tables and Calculations
Below calculations are made assuming the wet well is closed, and anaerobic conditions exist within the well.
The measurements of the concentrations were taken on dry and hot day.
Sample calculation for Table 1:
Column 1: Time at which the CO2 reading is taken
Column 2: Wastewater level in the wet well at the time of reading = 3.3 ft
Dimensions of the wet well:
Length = 36 ft
Width = 20.5 ft
Depth = 18.67 ft
Column 3:
Volume of wastewater present in the wet well
= Length * Width* Wastewater level in the wet well
= 36ft * 20.5ft * 3.3ft
= 2435.4 ft3
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Total volume of the wet well
= Length * Width * Total Depth
= 36ft *20.5 ft *18.67ft
= 13778.46 ft3
Column 4:
Volume of Air in the wet well
= Total Volume of the wet well – Volume of water in the wet well
= 13778.46 ft3 – 2435.4 ft3
= 11343.06 ft3
Column 5:
Volume of Air expelled out due to increase in the water level
= Difference between two consecutive air volumes
= 11343.06 ft3 – 10752.66 ft3
= 590.4 ft3
Column 6:
Concentration of CO2 in the Air in the Wet Well (ppm)
= 3000 ppm, CO2 reading in the Landtec GEM-2000 Plus instrument inside the wet well
Column 7:
Average hourly concentration of CO2 in the wet well
= Sum of two consecutive readings/2
= (3000 ppm +1000 ppm)/2
= 2000 ppm
Column 8:
Assuming CO2 density at standard temperature and pressure = 1.98 kg/m3 = 0.124 lb/ft3
CO2 expelled out of the wet well (lb)
= Volume of Air Expelled Due to Increase in the water level
* Average Hourly Concentration of CO2 in the Wet Well * CO2 density
= 590.4 ft3 * 2000 ppm *0.124 lb/ft3
= 590.4 ft3 * 2000 ft3/ft3 *10-6 * 0.124 lb/ft3
= 0.146 lb
Column 9:
Average Hourly Volume of Water present in the Wet Well (ft3)
= (2435.4 ft3 + 3025.8 ft3)/2
= 2730.6 ft3
Column 10:
CO2 Emitted per unit volume of waste water (lb/ft3)
= (CO2 expelled out of the wet well (lb)/Average Hourly Volume of Water present in the Wet Well (ft3))
= 0.146 lb/2730.6 ft3
= 5.3622E-05 lb/ ft3
Every hour 0.146 pounds of CO2is emitted out of the wet well. For one cubic feet of wastewater 5.3622E-05
lb of CO2 is emitted.
Similarly CO2 expelled out for the other wet well and H2S expelled out for the same wet well are calculated.
The density of H2S is 1.363 kg/m3 equal to 0.0851 lb/ft3.
For the Wet Well #2 lift station the H2S emissions could not be detected as the emissions were lower than the
least count of the instrument.
6. Health Impacts
6.1. Health effects of CO2
Exposure to CO2 can produce a variety of health effects. These may include headaches, dizziness, restlessness, a
tingling or pins or needles feeling, difficulty breathing, sweating, tiredness, increased heart rate, elevated blood
pressure, coma, asphyxia to convulsions and even frostbite if exposed to dry ice.
The levels of CO2 in the air and potential health problems are:
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M. Mudragaddam et al.
1) 250 - 350 ppm—background (normal) outdoor air level.
2) 350 - 1000 ppm—typical level found in occupied spaces with good air exchange.
3) 1000 - 2000 ppm—level associated with complaints of drowsiness and poor air.
4) 2000 - 5000 ppm—level associated with headaches, sleepiness, and stagnant, stale, stuffy air. Poor concentration, loss of attention, increased heart rate and slight nausea may also be present.
5) >5000 ppm—Exposure may lead to severe oxygen deprivation resulting in permanent brain damage, coma
and even death.
Also, CO2 in the atmosphere increases the Earth’s temperature which is known as “green house effect” (US
Department of Health Education and Welfare, Public Health Service, Centre for Disease Control, National Institute for Occupational Safety and Health, 1976).
6.2. Health effects of H2S
The levels of H2S in the air and potential health problems are:
1) 0.13 ppm—This is the odor threshold. Odor is unpleasant.
2) 4.6 ppm—Strong, intense odor, but tolerable. Prolonged exposure may deaden the sense of smell.
3) 10 - 20 ppm—Causes painful eyes, nose and throat irritation, headaches, fatigue, irritability, insomnia, gastrointestinal disturbance, loss of appetite, dizziness. Prolonged exposure may cause bronchitis and pneumonia.
4) 50 ppm—May cause muscle fatigue, inflammation and dryness of nose, throat and tubes leading to the
lungs. Exposure for one hour or more at levels above 50 ppm can cause severe eye tissue damage. Long-term
exposure can cause lung disease.
5) 100 - 150 ppm—Loss of smell, stinging of eyes and throat. Fatal after 8 to 48 hours of continuous exposure.
6) 200 - 250 ppm—Nervous system depression (headache, dizziness and nausea are symptoms). Prolonged
exposure may cause fluid accumulation in the lungs. Fatal in 4 to 8 hours of continuous exposure.
7) 250 - 600 ppm—Pulmonary edema (lungs fill with fluid, foaming in the mouth, chemical damage to lungs).
8) 300 ppm—May cause muscle cramps, low blood pressure and unconsciousness after 20 minutes.
9) 300 to 500 ppm may be fatal in 1 to 4 hours of continuous exposure.
10) 500 ppm—Paralyzes the respiratory system and overcomes victim almost instantaneously. Death after
exposure of 30 to 60 minutes.
11) 700—Paralysis of the nervous system.
12) 1000—Immediately fatal.
If caught in time, poisoning can be treated, and its effects are reversible. Some workers may experience abnormal reflexes-dizziness, insomnia and loss of appetite that lasts for months or years. Acute poisoning does not
result in death, may produce long-term symptoms such as loss of memory or depression, paralysis of facial muscles (Canadian Centre for Occupational Health and Safety, 1985).
7. Results and Discussion
At Wet Well # 1
On an average CO2 expelled out of the wet well every one hour = 0.139 lb
On an average H2S expelled out of the wet well every one hour = 0.002 lb
CO2 emitted per unit volume of wastewater (lb/ft3) = 4.3715E-05 lb/ft3
H2S emitted per unit volume of wastewater (lb/ft3) = 6.7177E-07 lb/ft3
At Wet Well #2
On an average CO2 expelled out of the wet well every one hour = 0.002 lb
CO2 emitted per unit volume of wastewater (lb/ft3) = 2.2545E-05 lb/ft3
It can be observed from Tables 1-3 that the concentrations of pollutants in the well change with temperature,
size of the well, and volume of water present in the wet well. The concentrations of the pollutants also depend
upon whether the wet well is directly exposed to the atmosphere or a superstructure exists above it. If a superstructure exists over a wet well, the conditions in the wet well may be considered as strictly anaerobic. Temperature is a very significant component for the H2S emissions, higher the temperature greater the gas emitted. CO2
emissions are less dependent on temperature when compared to H2S emissions. According to EPA, even at very
low concentration (<1 ppm) of hydrogen sulfide, it can corrode the electrical and mechanical devices and also
the concrete structures. The observed lift station exhibits higher hydrogen sulfide levels posing corrosion risks to
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M. Mudragaddam et al.
Table 1. CO2 emissions from the wet well.
LIFT STATION: Wet Well # 1
Volume of Air
Time (1)
in the Wet
Volume of
(Date: WW Level
Well (ft3) (Total
WW in the
06/12/09) in the Well
Volume of the
Wet Well
Dry
(ft) (2)
wet well =
(ft3) (3)
Weather
13778.46 ft3)
(4)
Volume of
Average
Average
Concentration
Hourly
Air Expelled
CO2 Expelled Hourly
of CO2 in the
Concentration out of the Wet Volume of
Due to
Air in the
of CO2 in the
WW present
Increase in
Well
Wet Well
in the Wet
water level
Wet Well
(lb) (8)
(ppm)
(6)
Well (ft3) (9)
(ppm) (7)
(ft3) (5)
CO2 Emitted
per unit volume of WW
(lb/ft3) (10)
10.00 AM
3.3
2435.4
11343.06
-
3000.0
-
-
-
-
11.00 AM
4.1
3025.8
10752.66
590.4
1000.0
2000.0
0.146
2730.6
5.3622E-05
12 NOON
5.2
3837.6
9940.86
811.8
4000.0
2500.0
0.252
3431.7
7.3333E-05
1.00 PM
3.6
2656.8
11121.66
0.0
3000.0
3500.0
0.000
3247.2
0
2.00 PM
5.5
4059.0
9719.46
1402.2
2000.0
2500.0
0.435
3357.9
0.00012945
3.00 PM
4.6
3394.8
10383.66
0.0
0.0
1000.0
0.000
3726.9
0
4.00PM
3.0
2214.0
11564.46
0.0
1000.0
500.0
0.000
2804.4
0
5.00 PM
4.5
3321.0
10457.46
1107.0
1000.0
1000.0
0.137
2767.5
0.0000496
Average CO2 Expelled out of the Wet Well per one hour =
0.139
CO2
emitted/unit
4.3715E-05
volume of
WW =
Table 2. H2S emissions from the wet well.
LIFT STATION: Wet Well # 1
Time (1)
Volume of Air
Average
Volume of
(Date:
Volume of in the Wet Well
Hourly
Concentration
H S Expelled
WW Level
Air Expelled
3
06/12/09)
WW in the
(ft ) (Total
of H2S in the Concentration 2
Due to
in the Well
out of the
Dry
Wet Well Volume of the
Air in the Wet of H2S in the
Increase in
(ft)
Wet Well (lb)
3
Weather
(ft )
wet well =
Wet Well
Well (ppm)
WW level (ft3)
Time
13778.46 ft3)
(ppm)
Average
Hourly
H2S Emitted
Volume of
per unit
WW present volume of
in the Wet WW (lb/ft3)
Well (ft3)
10.00 AM
3.3
2435.4
11343.06
-
19.0
-
-
-
-
11.00 AM
4.1
3025.8
10752.66
590.4
6.0
12.5
0.001
2730.6
0.00000023
12 NOON
5.2
3837.6
9940.86
811.8
69.0
37.5
0.003
3431.7
7.5492E-07
1.00 PM
3.6
2656.8
11121.66
0.0
89.0
79.0
0.000
3247.2
0
2.00 PM
5.5
4059.0
9719.46
1402.2
57.0
73.0
0.009
3357.9
2.5941E-06
3.00 PM
4.6
3394.8
10383.66
0.0
2.0
29.5
0.000
3726.9
0
4.00 PM
3.0
2214.0
11564.46
0.0
26.0
14.0
0.000
2804.4
0
5.00 PM
4.5
3321.0
10457.46
1107.0
40.0
33.0
0.003
2767.5
1.1233E-06
Average H2S Expelled out of the Wet Well per one hour =
140
0.002
H2S
emitted/unit
6.7177E-07
volume of
WW =
M. Mudragaddam et al.
Table 3. CO2 emissions from the wet well.
LIFT STATION: Wet Well # 2
Average
Volume of
Volume of
Volume
Time (1)
Concentration
Hourly
Air Expelled
WW
Air in the
of WW
(Date:
of CO2 in
Concentration
Level in
Wet Well (ft3)
Due to
06/12/09)
the Air in
in the
of CO2 in
the Well
(Total Volume Increase in
Dry Weather
the Wet
Wet Well
(ft)
the Wet Well
of the wet well water level
3
Well (ppm)
Time
(ft )
(ft3)
(ppm)
= 585.1 ft3)
CO2
Expelled
out of the
Wet Well
(lb)
Average
Hourly
CO2 Emitted
Volume of
per unit
WW present volume of
in the Wet WW (lb/ft3)
Well (ft3)
9.30 AM
1.2
76.3
508.8
-
3000
-
-
-
-
10.30 AM
2.1
133.6
451.5
57.3
1000
2000
0.014
104.96871
0.00013527
11.30 AM
2.3
146.3
438.8
12.7
1000
1000
0.002
139.95828
1.1273E-05
12.30 PM
2.0
127.2
457.9
0.0
0
500
0.000
136.77741
0
1.30 PM
1.5
95.4
489.7
0.0
1000
500
0.000
111.33045
0
2.30 PM
1.8
114.5
470.6
19.1
0
500
0.001
104.96871
1.1273E-05
3.30 PM
2.3
146.3
438.8
31.8
0
0
0.000
130.41567
0
4.30 PM
1.4
89.1
496.1
0.0
0
0
0.000
117.69219
0
Average CO2 Expelled out of the Wet Well per one hour =
0.002
CO2
emitted/unit
2.2545E-05
volume of
WW =
the Kenner City’s wastewater infrastructure.
8. Limitations
Practically no wet well is completely closed. There may be some leakages from the well which are not considered
in this approach of quantifying emissions. The emissions may change depending upon the composition of the
wastewater, pH, temperature, and residence time inside the wet well which should be further investigated.
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